1. Field of the Invention
The present invention relates to a thermal fixing device which is used in an image forming apparatus such as a copying machine, a printer, a facsimile machine or the like, and particularly a thermal fixing device including a resistance heating material layer, which generates heat from its surface when energized, i.e., when supplied with a current, for thermally fixing an unfixed image carried on a record medium onto the record medium by utilizing the surface-generated heat.
The invention also relates to a method of energizing a heating member in the thermal fixing device for raising the temperature of the heating member.
Further, the invention relates to an image forming apparatus provided with the thermal fixing device.
2. Description of the Background Art
The thermal fixing device in the image forming apparatus such as a copying machine, a printer, a facsimile machine or the like is generally provided with a thermal fixing roller. A record medium carrying an unfixed image such as a toner image is passed between the fixing roller and a backup member (generally, a pressure roller) under a pressure so that the unfixed image is fixed onto the record medium by heat and pressure.
A large amount of heat is required for heating the fixing roller.
Recently, it has been demanded to reduce a power consumption of the image forming apparatus during not only an image forming operation but also standby. Reduction in required electric power of the thermal fixing device is one of factors which contribute to reduction in power consumption of the apparatus.
For example, the thermal fixing device in the prior art, which applies the heat and pressure to the unfixed image carried on the record media for fixing the image onto the record media as described above, has such a structure that a heater such as a halogen heater is arranged within the thermal fixing roller for heating the fixing roller by heat radiated therefrom. In another structure, the fixing roller is heated by a heater of a sheet-like form, i.e., a so-called sheet heater, which is formed on a core roller of the fixing roller and made of a material generating heat when energized, i.e., when supplied with a current.
In the above thermal fixing device which uses the internal heater such as halogen heater or the like as the heat source, the electrothermal energy conversion efficiency is generally about 70%, and conversion loss of about 30% occurs.
In the above thermal fixing device which uses the sheet heater as the heat source, the electrothermal energy conversion efficiency is extremely high so that the power consumption of the image forming apparatus can be low, and thus the required electric power can be significantly reduced.
If such a sheet heater is used that has a capacity equal to that of the internal heater such as a halogen heater, a preheating time (i.e., a so-called warm-up time) which is required for heating the fixing roller to a predetermined temperature can be significantly reduced because the sheet heater has a high electrothermal energy conversion efficiency.
Accordingly, the sheet heater having a high electrothermal energy conversion efficiency can be effectively used as the heat source of the thermal fixing device from the viewpoint of reduction in required electric power.
However, the thermal fixing device employing the sheet heater has such features that a current which flows therethrough at the time of start of heating, i.e., when the fixing roller is cold (at the room temperature) is larger than that which flows when the fixing roller is warmed up. The feature that a large current flows at a low temperature is a general feature of the electrically conductive material, and the above heating layer has the same feature. An electrical resistance value rises with temperature.
In the thermal fixing device using the sheet heater as the heat source, therefore, a large current may flow even if only a small current will flow through the heat generating material layer after the fixing roller reaches the predetermined temperature suitable to fixing, because the electrical resistance value of the heat generating material layer is low during an initial stage of the temperature rising, i.e., when the temperature of the fixing roller is low.
For preventing this, such a structure may be employed that the heat generating material layer having a high electrical resistance value is employed for restricting the current flowing through the heat generating material layer in the initial stage of the temperature rising of the fixing roller. According to this structure, however, the current flowing through the heat generating body is reduced so that the temperature rising rate of the fixing roller is reduced.